1. Field of the Invention
The invention relates to a plasma-supported equipment for chemical gas phase deposition with an array type arrangement of microwave plasma electrodes and a control circuit, and also to a corresponding coating process.
2. Discussion of Related Art
The like is known from U.S. Pat. No. 5,017,404 (DE 38 30 249 C2). It is there provided that the individual plasma electrodes are arranged such that the plasma columns which are produced, overlap. The individual plasma electrodes are switchable and controllable independently of each other by means of the supplied electronic power, and in fact this is used to equalize edge effects or to produce a specific course of the coating properties. It is expressly a prerequisite that disturbing interference effects do not occur with the high frequency fields.
The application to the field of plasma pulse CVD technology is described, as are examples of microwave antennas and the like. The examples relate to two-dimensional arrays.
The said U.S. Pat. No. 5,017,404 is expressly incorporated by reference into the disclosure of this Application. The embodiment of apparatus systems are to be gathered therefrom and are suitable for the suitable control of the plasma electrodes for the embodiment of the invention.
In the case of high requirements on the homogeneity of large-surface layers it has been found that interference of adjacent microwave fields still disadvantageously appears, in contrast to the said document, when there is optimum shaping of the overlap of the plasma columns.
(European Patent) EP 0 420 117 A describes the disturbance due to interference in plasma CVD with excitation by a microwave array, and considers a stable operation to be impossible without their elimination. It is proposed to provide different polarizations, i.e., directions of the electric field vector, in adjacent microwave sources.
However, attaining homogeneous excitation behavior in the individual modules is obviously made more difficult here, since the crossed rectangular waveguides which are shown in the examples do not permit (this) because of the asymmetrical waveguide geometry.
Another kind of interference prevention would be the frequency displacement of adjacent plasma electrodes. Commercial microwave generators have marked frequency fluctuations and also high bandwidths, especially in pulsed operation, so that relatively large frequency differences would be required. However, different plasma-chemical modes of behavior can then no longer be excluded. In addition, microwave generators of optional frequency are not immediately available, since economical operation is only possible for permitted industrial frequencies.
The usual plasma CVD processes are characterized in that the reaction gas flows over the substrate during the whole coating period, and simultaneously energy which produces plasma is introduced into the reaction volume, so that the reaction gas and exhaust gas of an already successful (film-forming, etc.) reaction are either mixed in a manner which is not clearly arranged, or occur in different proportions at different locations of the substrate. The speed of a development, the properties of the coating (especially density, adhesion strength, and stability), and also the yield of reaction gas, are limited.
Such limitations are overcome by the application of the plasma pulse CVD process (PICVD process).
In this process, the energy which generates plasma is introduced in pulsed form, while the reaction gas flows continuously into the reaction space. It is typical for the PICVD process that the interval between pulses is matched to the time required to completely replace with fresh gas the gas volume over the substrate and implicated in the (film-forming) reaction. This time is dependent on several parameters, such as, for example, substrate size and shape, mass flow and temperature of the reaction gas, pressure in the reactor, and kind of gas inflow (e.g., nozzle form).
The process operates like a two-stroke motor; the interval between pulses, in which the used gas is replaced by fresh gas, follows the film-forming plasma pulse.
A further advantage of this process is the low temperature loading of the substrate, since the action of energy on it takes place only during the pulse period, and the substrate cools in the interval between pulses. It is thereby possible, vice versa, to use comparatively high energies during the pulse, and thus to deposit films with properties which otherwise only the solid material has.
The values for the pulse duration are typically between 0.1 and 10 ms, and for the duration of the interval between pulses, between 10 and 100 ms.
It is favorable to irradiate with microwave energy, since plasmas are then produced at gas pressures in the mbar region. Such gas pressures can be produced with comparatively little expense. The PICVD process can be advantageously applied, for example, for the internal coating of dielectric tubes from which, for example, preforms for optical fibers are produced (EP 0 036 191, DE 38 30 622, DE 40 34 211), for the application of IR-transparent dielectric mirrors to glass substrates of spherical surface shape (DE 40 08 405, DE 43 34 572), or for the deposition of planar thin film waveguides on glass or plastic (DE 41 37 606, DE 42 28 853).